Osteoblasts play key roles in the mechanisms of action of strontium ranelate

Abstract

Background and purpose: Strontium ranelate reduces fracture risk in postmenopausal women with osteoporosis. Evidence from non-clinical studies and analyses of bone markers in phase III trials indicate that this is due to an increase in osteoblast formation and a decrease of osteoclastic resorption. The aim of this work was to investigate, in human cells, the mechanisms by which strontium ranelate is able to influence the activities of osteoblasts and osteoclasts. Experimental approach: Human primary osteoblasts were used to examine effects of strontium ranelate on replication (thymidine incorporation), differentiation (Runx2 and alkaline phosphatase) and cell survival (cell counts and caspase activity). Osteoprotegerin (OPG) was measured by quantitative reverse transcription PCR (qRT-PCR) and ELISA and receptor activator of NFkB ligand (RANKL) by qRT-PCR and Western blot. As strontium ranelate has been proposed as an agonist of the calciumsensing receptor (CaSR), the involvement of CaSR in the effects of strontium ranelate on OPG and RANKL expression, and cell replication was examined using siRNA. Key results: Strontium ranelate increased mRNA and protein levels of OPG and suppressed those of RANKL. Strontium ranelate also stimulated osteoblast replication and differentiation and increased cell survival under stress. Knocking down CaSR suppressed strontium ranelate-induced stimulation of OPG mRNA, reduction of RANKL mRNA, and increase in replication, indicating the involvement of CaSR in these responses. Conclusions and implications:Our results demonstrate that osteoblasts play a key role in the mechanism of action of the anti-fracture agent, strontium ranelate by mediating both its anabolic and anti-resorptive actions, at least in part, via activation of CaSR.

An Akt-dependent Increase in Canonical Wnt Signaling and a Decrease in Sclerostin Protein Levels Are Involved in Strontium Ranelate-induced Osteogenic Effects in Human Osteoblasts

“…Akt phosphorylation was measured in HOBs that were pre-treated with 500 nM (10 ϫ IC 50 ) NPS2143 for 5 min followed by the addition of 2 mM strontium for 10 min. (21). In the current study pre-treatment of HOBs with 50 nM wortmannin abolished strontium-induced Akt-Thr 308 phosphorylation but had little effect on strontium-induced Akt-Ser 473 phosphorylation (Fig.…”

“…4A). Dual-phosphorylation of Akt may explain the prosurvival effect of strontium that was previously reported (21). Knock-out mouse studies have shown that of the three isoforms of Akt, Akt1 regulates the control of osteoblast survival (45), whereas Akt2 promotes induction of differentiation prior to Runx2 expression (46).…”

“…1A) (26). We and others have previously shown that strontium induces differentiation of osteoblasts, as measured by increases in alkaline phosphatase activity and RUNX2 mRNA expression (21,22), as well as osteocalcin (23). Here we found that strontium induced an increase in mineralization of dHOBs in a time-and concentration-dependent manner (Fig.…”

“…We and others have previously reported that treatment of primary human osteoblasts with strontium increases replication as well as expression of differentiation markers RUNX2 and alkaline phosphatase (21)(22)(23). These results complement in vivo studies in animals (24) and human subjects (6), which show increases in bone formation after treatment with strontium ranelate.…”

Osteoblasts in osteoporosis: past, emerging, and future anabolic targets

“…Interestingly, strontium ranelate, an approved antiosteoporotic drug, (27)(28)(29) was found to activate the CaSR in osteoblasts (30), resulting in activation of osteoblastic cell replication, differentiation, and survival (31,32). In OVX rats, strontium ranelate-treated animals exhibited increased bone formation and decreased bone resorption, resulting in prevention of trabecular bone loss, improved bone microarchitecture, and strength (33,34).…”